The highest RF powers are reserved for travelling wave tube (TWT) amplifiers and other magnetrons and klystrons. These allow power levels to be reached that are inaccessible with simple solid-state amplifiers, composed of transistors.
For medium power levels, there are also compact TWTs whose bulk is similar to that of basic transistor-based amplifiers.
However, with the progress made in the past few years in the field of transistors based on semiconductors of GaN and/or SiC type, these components allow solid-state amplifiers to be produced that gain ground on the power ranges covered by TWTs, with multiple extra advantages:                higher reliability and longer life;        a broader passband;        a reduced noise factor;        increased linearity;        a more conventional supply voltage.        
Solid-state amplifiers are produced in monolithic form or else in the form of hybrid components, up to a certain power level which currently extends up to a few hundred watts in the X-band. Beyond that, multiple elementary amplifiers (monolithic or hybrid) must be used in parallel to generate even higher powers and thus exceed the kW level.
For this, the signal is subdivided into multiple paths going to the various elementary amplifiers. The amplified signals are subsequently recombined into a single path.
Most often, the same type of device may be used as a power combiner or splitter. One role is switched to the other by swapping the sources and loads.
In order for the assembly to be energy efficient, insertion losses from these splitter-combiners should be optimized. From this perspective, structures based on waveguides are the most effective. They exhibit very low insertion losses and withstand very high powers, in particular in the form of rectangular waveguides.
However, these structures and, in particular, those using rectangular waveguides are very bulky and have limitations in terms of frequency bands which are linked to limited space and to the multiple modes that are propagated. For example, these structures are not suitable for a frequency band covering at least the X-band and for a space with a diameter of less than 50 mm.
Document U.S. Pat. No. 5,142,253 describes a solution with spatial power combining based on an oversized coaxial waveguide.
The term “oversized” means, in this instance, that the diameters of the coaxial structure are greater than the maximum dimensions required for the guide to operate in the single fundamental mode or for the cavities to be free of resonances in the useful band. By way of indication, a cylindrical guide has a first propagation mode (TE11) located above 10 GHz when its diameter exceeds 8.8 mm, and a coaxial line with a characteristic impedance of 50 ohms has multiple propagation modes (including TE11) above 10 GHz when its large diameter exceeds 13.3 mm. In summary and to give a rough indication, above 10 GHz a roughly cylindrical structure whose internal diameter exceeds 9 mm may pose problems of multiple propagation modes or parasitic resonances.
In order to keep production simple, this structure leads to oversized diameters over more than ⅔ of its length for frequencies above 10 GHz.
This solution requires substantial volumes to be occupied by RF absorbents, which necessarily increase losses. It is not possible to remove these absorbents without the possible occurrence of parasitic resonances that will decrease the performance of the structure.
This structure requires the assembly of numerous mechanical parts, thereby leading to an assembly lacking robustness.
This solution does not allow the simple interconnection of multiple elementary amplifiers, in particular with microstrip accesses (the most common configuration) distributed radially (the most practical arrangement for a cylindrical format). Moreover, they are highly mechanically complex to produce, in particular when compactness and coverage of high useful frequencies is sought.
Additionally, the specified performance, with insertion losses of 0.5 dB over a frequency band from 2.5 to 10 GHz, is not sufficient for the target applications.
Another, more recent, type of solution, also based on spatial power combining around a coaxial waveguide and better suited to radially arranged microstrip accesses has been proposed, for example, in document US2014/016788.
This solution makes it possible to split or recombine the power to/from multiple paths or elementary amplifiers, in a reduced space and over a broad frequency band.
However, the results of this solution show that the dimensions are too great and insertion losses are too high.
In this structure, the elementary amplifiers are arranged radially in the space between the two conductors that form a coaxial guide. This guide is substantially oversized so as to be able to house these amplifiers and the various parallel paths that carry out the required impedance transformations towards the microstrip ends.
The impedance transformations are carried out using antipodal lines, such as described in FIG. 5 of the document, that are adapted to coaxial lines. These structures have two metal tracks positioned on opposite faces of a substrate; these tracks have a structure that resembles a slot line, of high characteristic impedance, at one end and a microstrip line, of lower characteristic impedance, at the other end. One track progressively widens so as to produce the ground plane and the other track widens and then narrows to ultimately form the line of the microstrip structure.
The oversizing of the coaxial guide increases as the number of elementary amplifiers (or paths, in this instance the antipodal lines) increases: More space is necessarily required in order to house more amplifiers, but this is not the only reason, as when the number of amplifiers is increased, then, at constant impedance (maximum value) on the input of each path, the characteristic impedance of the coaxial guide is necessarily lower, thereby also leading to the guide being oversized in order to retain the same coaxial space. Specifically, it is necessary to increase the diameters of the coaxial line in order to obtain a lower characteristic impedance for one and the same space between the core (centre) and the body (perimeter) of the coaxial guide. As a result of the growing occurrence of propagation modes of higher orders and possible associated resonances, performance will decrease and bulk will increase as a function of the number of elementary amplifiers and the highest useful frequency.
Furthermore, as the diameters of the coaxial guide increase, paths or amplifiers must increasingly be used in order to minimize discontinuities. Stated otherwise, using a coaxial guide of smaller diameter allows broader bands of frequencies to be covered, with higher useful frequencies, or else requires a smaller number of elementary amplifiers. For example, by taking this line of reasoning to the extreme, a coaxial of sufficiently small size could even get by with a single amplifier, constituting a very dissymmetrical case that is favourable for the excitation of higher modes or resonances and which it is desirable to avoid in an oversized waveguide.
From this perspective, the structure described in document US2014/016788 requires a large number of amplifiers in order to provide a broad passband and raised high useful frequency.
The aforementioned drawbacks indicate that the performance and bulk of this structure are limited by these two antagonistic elements: on the one hand the number of amplifiers must be limited in order to limit bulk and provide sufficient performance at high frequency (insertion losses, efficiency, gain ripple), and on the other hand a minimum number of amplifiers is needed in order to provide sufficient RF performance (maximum power, passband).
Stated otherwise, the antagonism over the larger or smaller number of paths or amplifiers is due to the need to minimize their number so that the structure is compact and little affected by higher modes and other parasitic resonances (maximum useful frequency, insertion losses, ripples) and the need for enough amplifiers so that the transition between the antipodal and coaxial portion is efficient (maximum useful frequency, insertion losses, ripple, but also maximum power).
Additionally, the complexity of producing this structure is also substantial, and it becomes increasingly complex as more elementary amplifiers are added. Moreover, this structure requires the assembly of numerous mechanical parts, thereby leading to an assembly lacking robustness.
An aim of the present invention is to overcome the aforementioned drawbacks by proposing a single-mode splitter/combiner system that has a broad passband in the vicinity of a useful frequency, is compact and is produced from a reduced number of mechanical parts, thereby allowing the assembly to have increased robustness and mechanical strength.